Jon, following this blog, so far I'm not quite sure of the model.
Looking on Sarik site I'm assuming it's the 1/12th scale one, that has a 20 inch Hull. If that's right then the 800 motor is far too big, a 545 would be plenty for such a small model.
My large motor lanch Caroline is 56 inches, and weighing in at 13 to fourteen kilos is overpowered with an 800 motor, and my 4ft tug only uses a 540 motor and is really efficient.
Your boat isn't a speed boat, so some trial runs with the smaller motor will prove you don't need to waste money on a large motor.